The present invention relates to a system (both method and apparatus) for producing a "range image" of an area of the surface of an object, and a method for calibrating a system of this type. More particularly, the present invention relates to a range imaging system, and a method for calibrating the system, which are based on the principles of projective geometry.
Active, optical range imaging systems collect three dimensional coordinate data from visible object surfaces in a scene. These systems can be used in a wide variety of automation applications, including shape acquisition, bin picking, robotic assembly, inspection, gauging, mobile robot navigation, automated cartography, medical diagnosis (biostereometrics) and automated military applications. The range imaging sensors in such systems are unique imaging devices in that the image data points explicitly represent seen surface geometry as sampled surface points.
Range images are known by many other names depending on the context For example, range images have been referred to, variously, as a range map, depth map, 3-D image, 2.5-D image, digital terrain map (DTM), topographic map, surface profile, XYZ point list, surface distance matrix, contour map, and surface height map. Many techniques are known in the art for obtaining range images, but most active optical techniques are based on one of the following five principles:
(1) Radar measures "time of flight" transmission time to and from an object surface. The transmitted energy may be electromagnetic radiation or sonic waves. The transmitted energy may be pulsed, a continuous wave or an amplitude or frequency modulated wave.
(2) Triangulation measures two interior angles, angle AB and angle BC, and the baseline B of a triangle ABC, and then determines the length A and C from the viewing apparatus to the object surface. Basically, either the ambient light reflected from the object surface may be viewed from two angles, on opposite ends of the base line, or light may be projected onto the object's surface from one end of the base line and viewed or detected from the opposite end of the baseline. The light projected may be "structured" as a single point, straight line, multiple points, multiple lines, grid, circle or the like.
(3) The Moire method and holographic interferometry both use interference phenomena to determine the distance to an object surface. With Moire, one amplitude-modulated spatial signal (e.g. reflected light from a scene) is multiplied by another amplitude modulated spatial signal (the viewing grating) to create an output signal with surface depth information encoded as a phase difference. In holographic interferometry, coherent light from two separate laser beams, focused at a common surface point, is added and the surface depth is encoded in the detected phase difference.
(4) Lens focusing determines the distance to a point on an object surface viewed through a lens that is in focus. Similarly, with Fresnel diffraction of coherent light passed through a diffraction grating, exact in-focus images of the grating are formed at regular, periodic distance intervals whereas the grating images are out of focus in a predictable manner between the end points of these intervals.
(5) Stadimetry determines the distance to an object surface from the size of the image in a viewed scene. This technique requires that the size of the object be recognized and compared to the size of the viewed image.
All of the techniques heretofore known for producing range images require complex mechanical/optical/electronic means for implementation. Furthermore, the use of these techniques normally requires that the viewing geometry and dimensions, or in the case of stadimetry the object geometry and dimensions, be known in advance. Furthermore, complex calculations are normally required to produce the range image so that the range imaging apparatus is relatively costly.